A simple refrigeration system includes a compressor (e.g., a single compressor or multiple compressors arranged in parallel), a condenser, an expansion valve, and an evaporator which are interconnected by a plurality of pipes. The compressor moves a refrigerant (e.g., a gaseous refrigerant such as HFC404, HCFC22, or the like) through the system. Typically, the refrigerant exits the compressor as a high-pressure vapor. From the compressor, the high-pressure vapor flows to the condenser. At the condenser, the high-pressure vapor condenses back to a liquid thereby giving off heat that is removed from the system. From the condenser, the condensed refrigerant is conveyed to the expansion valve which decompresses the refrigerant. The decompressed refrigerant is conveyed to the evaporator where the refrigerant transitions to a vapor. The evaporator is typically located within an area desired to be refrigerated (e.g., a refrigeration case). As the refrigerant is evaporated within the evaporator, the temperature within the evaporator drops thereby causing heat from the area desired to be refrigerated to flow into the evaporator. In this manner, the evaporator performs a cooling function. From the evaporator, the refrigerant is circulated back to the compressor and the cycle is repeated.
Refrigeration systems operate more efficiently if the refrigerant exiting the condenser is cooled prior to being evaporated. Commonly, the refrigerant of a primary refrigeration system is cooled by using a secondary refrigeration system. This type of cooling is frequently referred to as "mechanical subcooling." If the secondary refrigeration system operates more efficiently than the primary system, there is an efficiency gain. This type of design is used often in commercial refrigeration systems for providing efficiency gain and for ensuring a solid column of refrigerant at the expansion device.
FIG. 1 illustrates a prior art refrigeration system 20 having mechanical subcooling. The refrigeration system 20 includes a primary system 22 and a secondary system 24. The primary system 22 interfaces with the secondary system 24 at a heat exchanger 26. At the heat exchanger 26, the secondary system 24 is used to subcool the refrigerant of the primary system 22.
The secondary system 24 includes a secondary compressor 28, a secondary condenser 30, a secondary expansion valve 32 and a secondary evaporator 34. The secondary evaporator 34 is positioned within the heat exchanger 26 and functions to subcool the refrigerant of the primary system 22.
The primary system 22 includes a primary compressor 36, a primary condenser 38, a receiver 40, a primary expansion valve 42, and a primary evaporator 44. FIG. 1 shows the refrigeration system 20 under normal operating conditions. At normal operating conditions, pressurized refrigerant vapor from the primary compressor 36 is condensed at the primary condenser 38. Condensed refrigerant from the primary condenser 38 is held within the receiver 40. From the receiver 40, the refrigerant flows through the heat exchanger 26 where the refrigerant is cooled. The cooled refrigerant is then conveyed to the primary expansion valve 42 where the refrigerant is decompressed. A liquid pump 43 adds pressure to the cooled refrigerant to prevent any flashing of the refrigerant to a vapor before reaching the primary expansion valve 42. Decompressed refrigerant from the primary expansion valve 42 is conveyed through the primary evaporator 44 where the refrigerant transitions to a vapor. The primary evaporator 44 is located within a region 48 desired to be cooled, and the evaporated refrigerant draws heat from the region 48. After exiting the primary evaporator 44, the refrigerant is cycled back to the primary compressor 36 and the sequence is repeated.
A problem with refrigeration systems such as the refrigeration system of FIG. 1 is the accumulation of ice within the evaporator (e.g., on the evaporator coils). To overcome this problem, most refrigeration systems periodically use a defrost cycle to melt ice accumulation within the evaporator. For example, one type of refrigeration defrost technique involves interrupting refrigerant flow through the evaporator. Another type of refrigeration defrost technique involves interrupting refrigerant flow through the evaporator in combination with resistance heating.
FIG. 2 shows a defrost cycle that uses hot gas from the compressor 36 to defrost the evaporator 44. In the defrost cycle, valve 50 is used to close fluid communication between the primary evaporator 44 and the intake of the primary compressor 36. Valve 52 opens fluid communication between the outlet side of the primary compressor 36 and the primary evaporator 44. In this manner, relatively hot defrost gas from the primary compressor 36 is pumped through suction line 54 and flows in a reverse direction through the primary evaporator 44. As the hot defrost gas flows through the primary evaporator 44, ice within the primary evaporator 44 is melted thereby cooling and condensing the defrost gas. The condensed refrigerant exits the primary evaporator 44 and bypasses the primary expansion valve 42 through bypass line 56. Bypass line 56 includes a one-way check valve 58 that allows refrigerant from the primary evaporator 44 to bypass the primary expansion valve 42, but prevents flow in an opposite direction. After bypassing the primary expansion valve 42, the refrigerant flows through solenoid valve 60 to return line 62. The return line 62 conveys the refrigerant back to the receiver 40. During the defrost cycle, the valve 60 closes fluid communication between the liquid pump 43 and the expansion valve 42.